Bulletin of the American Physical Society
2017 Annual Fall Meeting of the APS Ohio-Region Section
Volume 62, Number 18
Friday–Saturday, October 13–14, 2017; Miami University, Oxford, Ohio
Session D2: Poster Session: Condensed Matter |
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Chair: Mahmud Khan, Miami University Room: Kreger Hall 222 |
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D2.00001: Phonon Dispersion and Thermal Conductivity of Carbon Nanotubes (CNTs) Tuan Le, Mahfuza Khatun We will present phonon dispersion, phonon density of states, and thermal conductivity of carbon nanotubes (CNTs). These one-dimensional carbon nanotubes are extremely versatile and robust with their high electrical and thermal conductivities, and mechanical strengths. The theoretical analyses for the phonon dispersion relation and density of states are based on Dynamical Matrix method. The thermal conductivity and heat flux autocorrelation function are obtained using Green--Kubo formalism. The Nose Hoover thermostat and the Tersoff interatomic potential are incorporated in the simulator. Three open source code have been used in this investigation. These include: Visual Molecular Dynamics (VMD), Dynamical Matrix Code, and the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The numerical computation for thermal conductivity is based on equilibrium molecular dynamics (EMD) technique. Results of CNTs with different chirality, length, and temperature will be discussed. [Preview Abstract] |
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D2.00002: Hall Effect Studies of LPCVD grown $\beta $-Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ on Sapphire Danielle Smith, Said Elhamri, Adam Neal, Shin Mou, Hongping Zhao With its ultra-wide bandgap of 4.5-4.9 eV and large breakdown electronic field, $\beta $ - Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ has recently attracted attention because of its potential for next generation power electronics applications. The estimated breakdown field for $\beta $ -Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ is 8 MV/cm, much larger than 2.5 MV/cm for 4H-SiC and 3.3 MV/cm for GaN, which could enable power electronics with larger power density and greater efficiency [1]. Also, Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ has the potential to be more cost-efficient in mass production than other wide bandgap materials due to its ability to be synthesized through standard melt growth methods [2]. With this motivation, this study examines the electronic properties of $\beta $ - Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ via temperature dependent Hall effect measurements. Among several samples, the highest measured mobility was 34 cm2/Vs at room temperature and 40 cm2/Vs at 150K. These results indicate the potential of LPCVD grown Si-doped Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ for next generation semiconductor power electronics applications. [1] Applied Physics Letters 100, 013504 (2012) [2] Applied Physics Letters 103, 123511 (2013). [Preview Abstract] |
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D2.00003: Transmission and circular currents in a bi-output double-ring structure Robin Klause, Eric Hedin The electron transmission through a bi-output, double-ring nanoscale structure is studied as a function of external magnetic flux and system-leads couplings. Circular transmission currents in the rings are also calculated and shown to exhibit a strong dependence upon flux and electron energy. The symmetry of the ring system with respect to the source and drain couplings is varied through converting the double-ring system to single-ring system by breaking the inter-site bond shared between the two rings. A tight-binding model of the Schrodinger equation is used to analyze the electron transmission properties of the nano-scale ring structure (with six embedded quantum dots per ring). This system can also provide a model for a molecular naphthalene structure, connected in a bi-output configuration. We present calculations showing the quantum-mechanical circular transmission resonances and the system I/V characteristics as a function of external flux and system coupling parameters. [Preview Abstract] |
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D2.00004: Fabrication of Gated Corbino Discs on CVD Grown, Monolayer Graphene Michael Gasper, Ryan Toonen, Nicholas Varaljay, Robert Romanofsky, FĂ©lix Miranda Using commercially available, CVD grown, monolayer graphene deposited on thermally oxidized silicon wafers, we have fabricated a variety of test structures that are suited for on-chip probing. The test structures include Corbino discs with inner disc diameters ranging from 20 $\mu$m to 120 $\mu$m and gap lengths ranging from 55 $\mu$m to 105 $\mu$m. Additionally, a new type of structure that we call the twinaxial Corbino ellipse was fabricated on the same chip with minor axes set to 230 $\mu$m and major axes that varied from 276 $\mu$m to 460 $\mu$m. Inner disc electrodes with diameters ranging from 20 $\mu$m to 120 $\mu$m were placed at the foci of the elliptical devices. Our fabrication process involved the use a sacrificial aluminum layer, which prevented delamination during metal lift-off processes and kept polymer residues from degrading Ohmic contact quality. Gated current-versus-voltage traces from the Corbino discs revealed Dirac point operation corresponding to gate voltage values, which exhibited dependence on device dimensions and drain-to-source bias, ranging from 5 V to 15 V. [Preview Abstract] |
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D2.00005: Stress-optic coefficient of potassium terbium fluoride. William Poston, Said Elhamri, D. E. Zelmon Potassium terbium fluoride (KTF) is a material proposed to act as an optical isolator. Optical isolators are employed in high power laser systems in which reflected beams must be prevented from coupling into the pump laser. The stress-optic coefficient relates mechanical stress to the birefringence it induces. At high power, strains in the lasing material change the refractive index of the lasing material, which can distort the output beam. The stress-optic coefficient is thus a crucial measurement for the design of laser cavities that employ optical isolators. The stress-optic coefficient for KTF was obtained by applying mechanical stress to a sample and measuring the output intensity of a laser shone through the sample. The change in intensity with applied pressure is a result of the change in refractive index of the sample, through the stress-optic effect. The stress-optic coefficient of KTF was determined and is reported. [Preview Abstract] |
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D2.00006: Effect of Cu doping on the superconducting properties of ZrNi$_{\mathrm{2-x}}$Cu$_{\mathrm{x}}$Ga. Dharma Raj Basaula, Mahmud Khan ZrNi$_{\mathrm{2}}$Ga is one of the 19 Heusler compounds that exhibit a weakly coupled BCS type-II superconductivity. For all these materials, the van Hove singularities are believed to play an important role in driving the superconductivity. Partial replacement of Zr by Nb resulted in the broadening of the Van Hove singularity and introduced a high degree of disorder in Zr$_{\mathrm{1-x}}$Nb$_{\mathrm{x}}$Ni$_{\mathrm{2}}$Ga, causing a decrease in the T$_{\mathrm{C}}$ of the system. So, it is interesting to explore the superconducting properties of ZrNi$_{\mathrm{2-x}}$Cu$_{\mathrm{x}}$Ga. Since Cu has one more electron than Ni, the study will allow an understanding of the effect in superconducting properties due to an increase in the valence electron concentrations of the compound. Therefore, we have performed an experimental study on a series of ZrNi$_{\mathrm{2-x}}$Cu$_{\mathrm{x}}$Ga compounds by x-ray diffraction, electrical resistivity, dc magnetization and ac susceptibility measurements. In agreement with the parent compound, ZrNi$_{\mathrm{2}}$Ga, all samples crystallized in the cubic L2$_{\mathrm{1\thinspace }}$Heusler structure. For x $\le $ 0.25, all the ZrNi$_{\mathrm{2-x}}$Cu$_{\mathrm{x}}$Ga compounds showed superconducting properties with a decrease in T$_{\mathrm{c}}$. [Preview Abstract] |
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D2.00007: Crystalline Quality and Surface Roughness Optimization of Hetero-Epitaxial Titanium Nitride on Sapphire. H. A. Smith, Said Elhamri, B. M. Howe, L. Grazulis, M. Hill, A. N. Reed In this project we optimized the growth of hetero-epitaxial titanium nitride (TiN) on sapphire using controllably unbalanced reactive magnetron sputtering. TiN is a mechanically-robust, high-temperature stable metallic material; these properties make TiN a material of interest for robust electrodes and resilient plasmonics. We adjusted deposition parameters such as external coil current, temperature, nitrogen/argon ratio, growth time and magnetron power to optimize the crystalline quality and surface morphology of TiN. Post-growth, we measured crystallinity using X-ray diffraction, and surface morphology using atomic force microscopy. X-ray diffraction showed a single TiN peak with pendell\"{o}sung fringes; from these fringes we obtained a film thickness of \textasciitilde 50 nm. Atomic force microscopy showed a surface roughness of \textasciitilde 168 pm. Based on this characterization, we determined that the deposition parameters outlined in this presentation yielded (111)-oriented epitaxial TiN with minimal surface roughness. This optimization is a crucial first step in maximizing TiN's usefulness in the above mentioned applications. [Preview Abstract] |
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D2.00008: Millimeter-Wave, Spiral Phase Plates for Vortex Wave Generation Blake Amacher, Nitin Parsa, Michael Gasper, Ryan Toonen We have designed quasi-optical, spiral phase plates for the purpose of generating millimeter-wave Laguerre-Gaussian beams, which carry non-zero orbital angular momentum. Such beams will be used for experiments involving the interaction of electromagnetic waves, having frequencies ranging from 57 to 67 GHz, with solid state devices and materials. Two phase plates were fabricated from high density polyethylene. One was designed to yield the lowest mode of right-handed chirality (having an azimuthal modal index of $+$1), and the other was designed to yield the lowest mode of left-handed chirality (having an azimuthal modal index of -1). A customized programmable XYZ-stage was constructed with a sensor head that can simultaneously measure vertical and horizontal polarization components. A finite element analysis tool, ANSYS HFSS, was used to predict beam intensity patterns for comparison to measured results. [Preview Abstract] |
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D2.00009: Theoretical and experimental investigation of spontaneous emission from GeSn and SiGeSn alloys E. Ghanati, Z. Li, T. Tran, B. Wang, Y. K. Yoe, J. S Williams, I. Agha, J. Mathews Silicon-based infrared lasers have long been an area of interest, but the realization of such devices has yet to be achieved. Sn-based alloys present a possible solution to this problem. The binary alloy GeSn and the ternary alloy SiGeSn have band gaps in the infrared, and these materials are being grown on Si substrates. Thus these combinations have the capacity for being a cheap and available alternative for lasers/detectors in IR/MIR region. In this work we studied spontaneous emission of the binary/ternary structures both theoretically and experimentally. We use a modified van Roosbroeck-Shockley expression to model the spontaneous emission spectrum from GeSn and SiGeSn, which depends on the band gap, the strain in the material, the excess carrier density, and the temperature. The results of the model are compared to photoluminescence measurements obtained from GeSn and SiGeSn materials, and the model is found to match the experimental data in most cases. [Preview Abstract] |
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D2.00010: Voltage-Control of Millimeter-Wave Polarization Rotation from Magnetoelastic Membranes Nitin Parsa, Blake Amacher, Michael Gasper, Nathaniel Hawk, Ryan Toonen, Fang Peng Rotation of the linear polarization angle of millimeter-wave Gaussian beams (with frequencies centered around 61.25 GHz) has been observed from 30 micrometer thick membranes of silicone rubber infused with micrometer-scale nickel particles. The membranes were stretched across and fixed to piezoelectric ceramic annuli with a silicone-based adhesive. Applying a low-frequency AC voltage across the thickness of an annulus resulted in the excitation of a radial resonance, which in-turn caused the magnetoelastic membrane to expand and contract in the radial direction. We isolated the influence of nickel particle density modulation on rotation of linear polarization using a lock-in detection technique. We found that the amount of rotation scaled linearly with the amplitude of the low-frequency AC voltage signal. Efforts are currently underway to distinguish reciprocal rotation due to birefringence from non-reciprocal rotation due to the Faraday effect. [Preview Abstract] |
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D2.00011: Comparison of the bandgaps of Ga-containing and Ga-free type II superlattices via photoluminescence measurements. Logan Cordonnier, Said Elhamri, Heather Haugan, John Hudgins A 532 nm laser was used to obtain the photoluminescence spectra of GaSb/InAs and InAs/InAsSb type II superlattices at 5 K using a variety of power settings (10mW-2W). The main goal of these measurements was to obtain the bandgap of each investigated sample. The data was fit with a Gaussian/Voigt function. The maximum of the fitted curve corresponded to the bandgap of the sample. The bandgap energies of thee Ga-containing and Ga-free superlattices were then compared. Preliminary data suggests that while bandgap ranges were broader for Ga-free samples, indicating more defects, Ga-free materials are viable alternatives to Ga-containing materials for optical applications based on the intensity and consistency of their photo-response. [Preview Abstract] |
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D2.00012: Changes in magnetic and electric anisotropy of Co2FeAl Heusler alloy films due to oblique angle deposition Zeeshan Ali, Wei Zhou, Jeffrey Brock, Mahmud Khan, Khalid Eid We will present our studies of the structural, magnetic and transport properties of Co$_{\mathrm{2}}$FeAl Heusler alloy films fabricated by magnetron sputtering-oblique angle deposition. Oblique angle deposition is when the substrate is not placed facing the source of atoms during growth, but is tilted at an angle. Increasing this tilt/glancing angle caused significant changes in the properties of the film making it more porous, where the films grow in isolated nano-pillars rather than being smooth and continuous. This in turn lead to significant changes in magnetic anisotropy, where the coercivity of the films increased dramatically from 30 Oe to 400 Oe, as the deposition angel was increased. The electrical resistivity of the films also increased steeply, especially for angles larger than 60\textdegree. [Preview Abstract] |
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D2.00013: Using ZnO:Sb micro-wires as oxygen sensors Brian Knauf, Tej Poudel Chhetri, Nada Masmali, Zeeshan Ali, Herbert Jaeger, Lei Kerr, Khalid Eid We fabricated ZnO:Sb micro-wires using a simple thermal technique and then studied the dependence of their electrical resistance on light, temperature and the abundance of ambient oxygen. While this work demonstrates that individual wires are quite sensitive to oxygen gas flow, both temperature and light illumination strongly affect their oxygen gas sensitivity and stability. Operating these ZnO:Sb oxygen sensors at 200\textdegree C gives the highest response to oxygen, yet a vanishingly small effect of light and temperature variations. The underlying physics and the interplay between these effects will also be discussed. [Preview Abstract] |
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D2.00014: Hydrothermal Synthesis of Alkali Tantalate Nanocubes Sunil Karna, Priya Karna, Dipesh Ghimire A set of experiments was conducted to synthesize alkali tantalates in energy efficient hydrothermal process. Lithium, sodium, and potassium tantalum oxide nanocubes were synthesized at $140^{o}C$ for 15 hours in rich alkaline atmoshphere. The final products were obtained as a perovskite phase of sodium tantalate, and a defect phase pyrochlore of potassium tantalum oxide and a lithium tantalum oxide. The size of nanocubes were 25 nm, 90nm, and 50 nm for lithium tantalum oxide, sodium tantalate, and potassium tantalum oxide nanocubes, respectively. The morphological, compositional, structural, thermal, and photophysical properties of as-synthesized nanocubes were characterized using scanning electron microscope (SEM), x-ray powder diffraction (XRD), thermal gravimetric analysis (TGA) techniques, and UV Spectrometer. [Preview Abstract] |
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D2.00015: Electrical properties of Ti/SiC Schottky barrier diodes Tom Oder, Krishna Kundeti, Sundar Isukapati, Nicholas Broucki Silicon carbide Schottky barrier diodes were fabricated with Ti Schottky contacts. The contacts were deposited at different temperatures ranging from 28 $^o$C to 900 $^o$C using a magnetron sputtering deposition system. The diodes were then annealed at 500 $^o$C in vacuum for up to 60 hours. Diodes with the contacts deposited at 200 $^o$C and annealed for 60 hours had the optimum current-voltage characteristics consisting of large barrier height of 1.13 eV and ideality factor of 1.04. These diodes also had a very low leakage current of 6.6 x 10-$^8$ A at a reverse voltage bias of 400 V. The X-ray diffraction analysis of the Ti/SiC contacts revealed the formation of TiC, Ti5Si3 and Ti3SiC2 at the interface. The improved properties for diodes with contacts deposited at 200 $^o$C could be related to formation of reaction products possessing high work functions. These improvements could provide significant gains in performance of 4H-SiC Schottky diodes where Ti is a common metal contact. [Preview Abstract] |
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